This invention relates to a process for preparing 4,6-dinitroresorcinol in one step from resorcinol.
4,6-Dinitroresorcinol is used to prepare 4,6-diaminoresorcinol, which is a precursor to polybenzoxazoles (PBOs), polymers which are useful as insulators, solar arrays, and cut-resistant gloves. PBOs can be prepared by reacting 4,6-diaminoresorcinol with bisacids, bisacid halides, bisesters, bisamides, or bisnitriles. See Wolfe in Mark et al., The Encyclopedia of Polymer Science and Engineering, Volume 11, pp. 601-635, Wiley-InterScience Publication, New York, 1988.
Efforts to prepare 4,6-dinitroresorcinol in one step from resorcinol and nitric acid in yields exceeding 30 percent have been unsuccessful due to the formation of high levels of undesirable by-products, such as 2,4-dinitroresorcinol and 2,4,6-trinitroresorcinol (styphnic acid). Improved yields of the desired 4,6-isomer have been obtained by introducing bulky protecting groups at the 1- and 3-positions of resorcinol, thereby inhibiting nitration at the 2-position. For example, Lysenko et al. (U.S. Pat. No. 4,982,001, herein incorporated by reference) describes the preparation of 4,6-dinitroresorcinol from resorcinol through a 1,3-bis(methylcarbonato)benzene intermediate. Similarly, Schmitt et al. (J. Org. Chem. 1988, 53, 5568, herein incorporated by reference) describes the preparation of 4,6-dinitroresorcinol in 44 to 66 percent yield from resorcinol diacetate. The steric hindrance of these intermediates causes nitration to take place mostly at the 4- and 6-positions, so that the desired 4,6-diaminoresorcinol can be prepared upon hydrolysis and hydrogenation. Unfortunately, a significant degree of undesirable nitration still occurs at the 2-position of the intermediate, and there is the danger of forming the potentially explosive styphnic acid.
It would be desirable to prepare 4,6-dinitroresorcinol in high yields and at high concentrations in one step from resorcinol without requiring any intermediate protected species.